1. Field of the Invention
This invention relates to a process for the production of prepreg sheets used in the manufacture of electronic printed circuit boards, and in particular to prepreg sheets which are free of voids.
2. The Prior Art
In the manufacture of electronic devices, such as computers, printed circuit boards are widely used to support discrete electronic components and to provide the electrical circuitry between the components. Commercial electronic computers have become more powerful since their introduction, yet they have been reduced in physical dimensions from room size to desk size. As their size has decreased and the number of interconnections due to more powerful logic has increased, the printed circuit boards used have become denser and more complex. Today's printed circuit boards can be extremely dense, with very small geometrics and with many layers.
The printed circuit boards have a central core typically a dielectric material, such as a composite of a reinforcing cloth formed from a material such as glass fiber and a thermosetting resin such as an epoxy resin referred to in the art as a "prepreg". The term "prepreg" is derived from the face that the reinforcing cloth, e.g., a glass fiber cloth, is preimpregnated with a volatile solution containing primarily a thermosetting resin such as an epoxy resin, catalysts and/or curing agents for the resin. The prepreg has applied on at least one surface thereof a layer of conductive material such as copper. The layer or layers of conductive material are etched or otherwise processed to provide circuits of predetermined geometrical configuration. The individual printed circuit boards may then be laminated to form a multilayer structure having the etched metal circuitry sandwiched between prepreg layers.
In the manufacture of the prepreg, the reinforcing cloth suitably a glass fiber cloth is impregnated with the thermosetting resin, e.g., epoxy resin solution by passing the cloth through a bath of the resin solution. After removal from the impregnation bath, the prepreg is heated to remove the solvent. When the resin is an epoxy resin, the prepreg is heated at about 100 to 150 degrees C. to remove the solvent and to partially cure, i.e., advance the epoxy resin to the B-stage. Thereafter the prepreg is laminated on either side with a conductive material such as a thin, e.g. 1.4 mils thick, copper foil to form an individual circuit board, or the individual boards once the circuitry is defined, may be laminated, using the prepreg as the laminating layer to form multilayer structures. One drawback to the prior art processes used for the manufacture of the prepreg is that under the conditions used for commercial manufacture, i.e. high line speeds and the temperatures used for solvent removal, the prepreg sheet product contains a large number of voids, generally appearing as spherical voids in the resin matrix and as cylindrically shaped (i.e. cigar shaped) voids between the filaments of the reinforcing cloth. The term "void" as used herein means an empty space having a diameter larger than 1 to 2 mils. A printed circuit board to be acceptable for commercial use should not have a void content in excess of 10 per layer after lamination. The presence of voids in the prepreg is frequently accompanied as when a catalyst such as dicyandiamide is incorporated in an epoxy resin impregnant, by the formation of catalyst crystals which appear under polarized light, as white spots in the epoxy matrix and between the glass filaments.
The presence of voids makes it extremely difficult for the resin of the prepreg material to flow completely into the deep corners formed between circuit board and printed circuit details due to the air entrapped in the voids. The voids also promote the deterioration of laminate integrity at the interfaces between adjacent layers and lead to metal, e.g. copper, migration and the formation of short circuits between plated through holes and power planes present in the printed circuit board.
The formation of catalyst crystals in the resin matrix and between the cloth filaments degrades the moisture resistance of the prepreg and causes non-uniform curing of the thermosetting resin.
It is therefore a primary objective of the present invention to provide a simple means whereby the formation of voids and catalyst crystals in prepregs is eliminated or substantially reduced.